pid loop optimisation: the benefits of using profibus pa instrumentation - peter thomas

PID Loop Optimization

& the benefits of using

PROFIBUS PA

instrumentation

Peter Thomas

Control Specialists Ltd

www.controlspecialists.co.uk

Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation – Apr 2014

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Objectives

• To gain a basic understanding of the principles of PID Loop Optimisation.

• To understand why “Loop Tuning” is often not the solution to achieving (or restoring) stability in a process control loop.

• To understand how PROFIBUS PA instrumentation can assist in Loop Optimisation.

Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014

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Loop Optimization or Loop Tuning?

• PID Loop Optimisation describes a series of activities that lead (or should lead) to a reduction in variance in a process control loop.

• “Loop Tuning” specifically refers to the adjustment of three parameters in a process controller (P)roportional, (I)ntegral and (D)erivative to reduce variance.

• Variance usually shows itself in the form of the process variable (PV) oscillating around the desired value (i.e. the Setpoint SP).

• A commonly held view is that “Loop Tuning” is the answer to all control loop variance issues.


Measure Decide Action

Noise

Variability Tuning

Controller Design

Hysteresis

Stiction Non-Linearity

Operator Changes

Loop not In Normal

Disturbances

Valve at Limit

Interactions

Typical Control Loop Issues


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Common Loop Tuning Methods

• Gut Feel / “Seat of the Pants” Tuning. • Copying PID values from another controller. • Model-Based Tuning Rules (Ziegler-Nicholls,

Cohen-Coon, Lambda). • Auto-Tuning Controllers. • Software-based Loop Optimisation Software

(ExperTune PID Loop Optimizer, Protuner)


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Common Loop Tuning Methods (1)


Method Comments

Gut Feel / “Seat of the Pants” tuning.

Success is very dependent upon the experience of the person doing the work.

The objective is usually to get the loop responding as required with no importance being placed on the actual PID values that have achieved this. This approach often leads to hiding loop-related issues.

No consistency in the approach and unlikely to look for problems outside of the controller itself.

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Method Comments

Model-Based Tuning Rules (Ziegler-Nichols, Cohen-Coon, Lambda).

Ziegler-Nichols is the most well known of these. All assume that the open loop response of the loop approximates to a first-order.

No use on Level Loops

Some are better than others when deadtime is an issue, i.e. it is not always obvious which is the most appropriate to use.

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Method Comments

AutoTuning Controllers Quick and simple way to tuning a loop.

They accept the loop response as found with no regard to the causes of the instability.

Can hide loop-related issues by continually retuning to overcome them.

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Method Comments

Software-Based Loop Optimisation Sotware Tools (ExperTune / Protuner)

They promote the idea that loop optimisation is the optimisation of the entire loop, not just the parameters of the process controller.

They uncover loop related problems that can make tuning difficult and/or impossible.

The PID values they calculate are at least equal to, and often better, than the values you are likely to get from the most appropriate model-based tuning rule.

Require OPC Access to the PV and OP of each loop.

Control Valve Problems

• Oversized • Non-Linear Response • Hysteresis / Deadband • Stiction • Poor Rangeability • Actuator Problems

Control valves are estimated to be the cause of 30% of control problems


Inherent Characteristics

A plot of valve opening verses flow Assuming constant pressure drop across valve

On/Off applications

Most Common


What is Valve Hysteresis?

A failure of the valve to reposition

itself correctly following a reversal in the control signal.


Causes of Hysteresis

• Undersized actuator • Seal friction in rotary

valves • Friction in plug valves • Inadequate air supply • Loose or worn actuator

linkages • Loose linkages to

positioner • Improper calibration of

positioner


Valve Stiction (Stick – Slip)

Time (s)

Stick

Slip Posi

tion

(%

)


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Demonstration


We will use PID Loop Optimizer and Loop Simulator from ExperTune to demonstrate the following:

• Tuning a simple flow loop. • The effect of over-filtering the process variable. • The effect of valve oversizing. • The effect of incorrect valve characterisation. • The effect of valve Hysteresis / Stiction.

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Demonstration


Demonstration

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PROFIBUS PA Transmitter Benefits

• Transmitter range easily adjusted. • Allows you to linearize the signal –

e.g. level in a cylindrical vessel. • Actions to be taken on specific types

of PV fault can be programmed into the PLC by monitoring the status byte of the PV.

• PV Filter can be monitored at the SCADA and easily changed


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PROFIBUS PA Valve Positioners

• Can be easily tuned – The positioner can be considered as the secondary loop of a Cascade Pair.


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• Valve Characteristic easily changed.

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• Can provide a time-delayed alarm when the actual position differs from the demanded – identification of Hysteresis / Stiction.

• Able to advise on the number of valve reversals – valve working too hard.

• Able to accumulate total valve travel – opportunity to reduce costs due to consequential use of compressed air.

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Summary

• Loop Tuning is only part of the overall process of optimising a PID Loop.

• Transmitter filtering and ranging can affect the performance of a process control loop.

• Control Valves are a common source of control loop problems.

• PROFIBUS PA instrumentation can dramatically reduce the time taken to optimise a process control loop and allows loop-related issues to be identified as a cause of variance.


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Questions?

Peter Thomas Control Specialists Ltd

[email protected]

LinkedIn – http://www.linkedin.com/company/2198880

www.con Tel +44(0)1925 824003 Mob +44(0)7971 405871


pid loop optimisation: the benefits of using profibus pa instrumentation - peter thomas

Technology

process control loop

entire loop

pid loop optimization

control loop variance

common loop tuning methods

loop related problems

looprelated issues

open loop response